Metal containers are typically coated with a protective coating to prevent damage to the container surfaces or contamination of the material packaged inside. Conventional container coatings may be derived, for example, from a formulation that includes trimellitic anhydride, a diol crosslinker, and an epoxy resin. These epoxy formulations described are applied as dispersions in a volatile organic solvent and then baked to form a lacquer-like coating on a metal substrate. Unfortunately, however, these volatile organic compounds (VOCs) are released into the atmosphere during the baking process, and may remain in the coating and degrade a product stored in the container.
To reduce VOC emissions, a heat curable coating for metal food contact surfaces may be derived from a water dilutable dispersion containing an epoxy resin. An effective water based coating may be obtained by adducting sufficient bisphenol to a diglycidyl ether of a bisphenol to react with all epoxy groups. The resulting oxirane defunctionalized adducts are then made water-soluble by reaction with an anhydride. The resulting dispersions are made thermoset using a conventional aminoplast crosslinking agent, such as highly butylated urea formaldehyde. Unfortunately, however, condensation products and by-products of these crosslinking agents, such as methanol, butanol, and formaldehyde, are released into the atmosphere during the baking process.
There is a need in the packaging coatings industry for improved coatings (e.g., packaging coatings) that protect the packaged goods from contamination and do not release harmful compounds into the atmosphere during the baking process. Changes in food processing and environmental regulations continue to prompt manufacturers to develop new coating formulations with superior safety and processing characteristics compared to existing formulations. The present invention provides such formulations. In particular, the present invention provides improved resin compositions that comprise the reaction product of an epoxy resin and an anhydride. In preferred embodiments the resin composition""s epoxide equivalent weight (EEW) has not been significantly changed, compared to the unreacted epoxy resin, to an extent that would cause any undesirable gelling or crosslinking of the resin. The reaction may preferably be conducted in an aprotic solvent.
In certain embodiments, the resin and coating compositions of the present invention do not require additional crosslinkers to form a crosslinked network. In addition, the resin compositions of the invention may be emulsified in water to form water based coating compositions that have reduced VOC emissions during bake. The self-crosslinking coating compositions of the invention are stable in aqueous media, have stability and curing profiles that comport with production scale food and beverage packaging applications, adhere well to metal, and are resistant to leaching, corrosion, and other forms of degradation.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
In one embodiment, the invention includes a resin composition that is the reaction product of an epoxy resin and an anhydride, wherein the reaction product has oxirane groups available to participate in further crosslinking reactions. The resin composition""s epoxide equivalent weight (EEW) preferably has not been significantly changed, compared to the unreacted epoxy resin, to an extent that would cause any undesirable crosslinking of the resin. This resin may be used as a coating, or may be formulated with other components to create a coating composition.
The epoxy resin includes a backbone with pendant oxirane and pendant hydroxyl functional groups. In general, the reaction of the epoxy resin and the anhydride takes place in an organic medium under conditions selected such that the oxirane groups remain substantially intact and the pendant hydroxyl groups react with the anhydride to form ester linking groups on the backbone. The ester linking groups have pendant carboxyl functional groups.
The epoxy resins may vary widely depending on the intended application. The epoxy resin includes a pendant oxirane group and a pendant hydroxyl functional group on a backbone. A representative epoxy resin is shown in Formula 1: 
Where B represents the backbone, X is an oxirane group, Y is a hydroxyl functional group, and n and m are independently at least 1, preferably at least 2.
In one embodiment, the epoxy resin is a reaction product of an epoxide and a dihydroxy compound. The dihydroxy compound used to make the epoxy resin may vary widely depending on the intended backbone structure needed in the epoxy resin. Preferably, the dihydroxy compound is selected from bisphenol A, bisphenol F, biphenol, resorcinol and the like, and bisphenol A is particularly preferred. Commercially available epoxy resins that are suitable for the present invention include those available under the trade designations EPON 1001, 1004, 1007, 1009, and 2004 resins from the Shell Chemical Co., Houston Tex. Preferred epoxy resins have a number average molecular weight of about 1,000 to about 10,000 and an epoxy equivalent weight of about 500 to about 5,000. Most preferred epoxy resins have an average number molecular weight of about 1,000 to about 8,000 and an epoxy equivalent weight of about 500 to about 5,000.
The epoxy resin is reacted with an anhydride to form a resin composition. Suitable anhydrides may vary widely depending on the epoxy resin selected and the reaction conditions. Examples of useful anhydrides include succinic anhydride, methyl succinic anhydride, tricarballylic anhydride, phthalic anhydride, hexahydrophthalic anhydride, trimellitic anhydride, itaconic anhydride, and maleic anhydride. Dianhydrides, such as, for example, benzophenone tetracarboxylic dianhydride (BTDA) or pyromellitic dianhydride, may also be used, and may increase cure rate and form a more densely crosslinked reaction product. When dianhydrides are used care should be taken to ensure that no undesirable gellation of the resin occur.
To prepare the resin compositions of the invention, the epoxy resin and the anhydride are reacted in a liquid medium under reaction conditions such that the reaction between the anhydride and the hydroxyl functional groups is substantially preferred over the reaction between the anhydride and the oxirane groups. The progress of the reaction can be monitored through methods such as NMR, IR, gas chromatography, or other methods known in the art. The resulting reaction product has oxirane groups available for further crosslinking reactions, and the reaction product""s epoxide equivalent weight (EEW) preferably has not been significantly changed, compared to the unreacted epoxy resin, to an extent that would cause any undesirable gelling or crosslinking of the resin. Preferably, the resulting reaction product""s EEW is no more than 50% higher than that of the starting epoxy resin. More preferably, the resulting reaction product""s EEW is no more than 30% higher than that of the starting epoxy resin. Most preferably, the resulting reaction product""s EEW is no more than 15% higher than that of the starting epoxy resin.
The anhydrides react with the pendant hydroxyl groups on the epoxy resin to generate ester acids. This reaction is represented in formula 2 below: 
Where B is the resin backbone, X is an oxirane group, Y is a hydroxyl group, L is an ester linking group, Q is a reactive carboxyl functional group, m and n are as previously defined, p is at least 1, preferably 2, and the sum m-p is at least 0.
The liquid medium used to prepare the resin compositions of the invention is preferably selected from aprotic solvents such as ketones, ethers, aryl ethers, ether esters and alkyl ethers, aromatic hydrocarbons (e.g., toluene, xylene, etc.), used alone or as mixtures. Suitable solvents or solvent mixtures have between about 2 and about 8 carbon atoms. Suitable ketones or esters include aliphatic compounds containing between 3 and 8 carbon atoms, such as, for example acetone, diethyl ketone, methylethyl ketone, methylpropyl ketone, methylbutyl ketone, methylamyl ketone, methylhexyl ketone, ethylpropyl ketone, ethylbutyl ketone, ethylamnyl ketone, dioxane, tetrahydrofuran (THF), and methoxy acetone. Dialkyl ethers of alkylene glycols and polyethylene glycols, such as glyme, diglyme, and the like, may also be used. Suitable alkyl ethers of diethylene glycol may contain between 1 and 4 carbon atoms in the alkyl group. Preferred liquid media include 1-methoxy-2-propranol acetate and methyl ethyl ketone (MEK).
Reaction temperatures for synthesizing the resin compositions of the invention are typically less than about 120xc2x0 C. The preferred temperature range is from about 40xc2x0 C. to 120xc2x0 C. More preferably, the temperature range is from about 60xc2x0 C. and 100xc2x0 C., and most preferably, from about 70xc2x0 C. to 80xc2x0 C. Typically reaction times are less than about 20 hours. Preferably, they are between about 4 and 16 hours, and most preferably, between about 5 and 15 hours.
A catalyst is preferably used to prepare the resin compositions of the present invention. While not wishing to be bound by any theory, the catalyst is believed to selectively enhance the formation of the ester linking groups between resin backbones via reaction of the hydroxyl group with the anhydride group, and limit the reaction between the carboxyl functional groups on the opened anhydride and the oxirane groups on the resin backbone. Preferably, the catalyst is a tertiary amine. Suitable catalysts include, but are not is limited to methyl diethylamine, triethyl amine, dimethyl propyl amine, methyl dipropylamine, tripropyl amine, methyl diisopropyl amine, methyl dibutyl amine, ethyl dibutyl amine, tributyl amine, N,N diethyl benzyl amine, 1,4-diazabicyclo(2,2,2) octane (DABCO), 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU). Most preferably, the catalyst is a tertiary benzylic amine, such as N,N-dimethylbenzyl amine, N,N diethylbenzyl amine, and the like.
The catalyst is preferably used in an amount about 0.01 to 0.5 wt %, more preferably about 0.02 to 0.3 wt %, and most preferably about 0.03 to 0.1 wt %, based on the total weight of reactants.
The resin compositions of the invention preferably have an epoxy equivalent weight (EEW) of about 500 to about 5,000, more preferably from about 1,000 to about 4,000. The resin compositions preferably have a weight average molecular weight (Mw) of about 2,000 to about 20,000, more preferably from about 2,000 to about 15,000. The resin compositions preferably have a number average molecular weight (Mn) of about 1,000 to about 8,000, more preferably from about 2,000 to about 8,000.
The resin compositions of the invention preferably have an acid number (expressed in conventional units of mg KOH/g) of about 10 to about 75, more preferably from about 15 to about 55, and most preferably from about 15 to about 30.
The resin composition of the invention may be used as a coating composition. These coating compositions may include other additives and agents to provide formulations that can be applied to substrates such as, for example, metal containers. These materials may include additives such as carriers, emulsifiers, pigments, fillers, anti-migration aids, curing agents, coalescents, wetting agents, biocides, plasticizers, crosslinking agents, antifoaming agents, colorants, waxes, anti-oxidants, or combinations thereof. The coating composition may be applied to a substrate and subsequently baked to form a fully cured coating.
However, to reduce the amount of VOCs evolved during the baking step, in another embodiment of the invention the resin composition is mixed with water and a water-soluble base such as, for example, a tertiary amine, to form a water-based coating composition. Suitable water-soluble tertiary amines include, for example, dimethylamino ethanol. To form the water-based coating composition, sufficient water is preferably added to emulsify the resin composition and create a phase inversion with the water forming a continuous aqueous phase and the resin composition forming a discontinuous phase suspended within the aqueous phase. While the rheology of the emulsion depends on the hydrophobicity of the liquid organic medium, typically the particles of the resin composition are small particles preferably having an average particle size of less than about 0.5 micron, more preferably less than about 0.3 micron, and most preferably less than about 0.1 micron. The water based coating composition of the invention, which typically has a pH of about 6 to about 8, appears translucent, and has a shelf life of at least about 3 months to about 6 months and longer.
Suitable crosslinking agents that may be used in the coating compositions of the invention include, for example, amino resins, phenolic resins, blocked isocyanates, etc.
Some specific examples of suitable amino resins include fully or partially alkylated melamine-formaldehyde resins, benzoguanamine-formaldehyde resins, urea-formaldehyde resins, and Glycoluril-formaldehyde resins. More specifically, suitable crosslinkers include commercial materials available from Cytec industries under the trade designations Cymel 303, Cymel 325, Cymel 1123, Cymel 1125, Cymel 1156, Cymel 1170, Cymel 5010, Beetle 80, and Beetle 1054, and those available under the trade designation Mepranel MF-800 from Hoechst, etc.
Some specific examples of phenolic resins that may be used in the coating compositions of the invention include phenol-formaldehyde resins, cresole-formaldehyde, Bisphenol A- formaldehyde resins, and un-alkylated, partially-alkylated or fully-alkylated formaldehyde resins. Some more specific examples include those available from Oxychem under the trade designations Varcum 94-607, Varcum 29-116, Varcum 29-159, those available under the trade designations HRJ11206, HRJ2527 from Schenectady, those available under the trade designation EP 560 from Solutia, and those available under the trade designation Uravar FB210 from Schenectady. Blocked isocyanates can be aliphatic, cyclo-aliphatic, or aromatic poly-functional isocyanates, blocked with, for example, MEK-Oxime, epsilon-caprolactam, uretedione, alcohols, glycol ethers, etc. More specific examples include compounds available from Degussa under the trade designations Vestanat B1358, Vestanat B1370, Vestagon B 1530, and Vestagon BF 1540.
If desired, the coating composition of the present invention may optionally comprise an additional resin, such as a poly-hydroxy or phenoxy group containing resin. For example, epoxy or phenoxy resins having two or more hydroxy groups may be utilized. Typically, such resins will have epoxy or phenoxy end groups. The presently preferred epoxide equivalent weight (EEW) of such optional resins is greater than about 1,000. Examples of suitable such additional resins include those available from Shell under the trade designations EPON 1001, 1004, 1009 and those available under the trade designation DER 684 EK40 from Dow Chemical. In one embodiment the coating composition comprises (i) 20 to 100 parts by weight of the resin of formula 2; (ii) up to 80 parts by weight of a suitable additional resin having epoxy or phenoxy groups; and (iii) up to 20 parts of a suitable crosslinking agent.
The coating compositions of the invention may be applied to a substrate by any procedure known in the art, including spray coating, roll coating, and the like. Preferably, the coating is applied to a metal sheet or coil or the interior of a metal can using an airless spray. After application to a substrate, the coating composition is heated in a baking process to form a cured coating. During the bake, the ester acids react with the hydroxyl groups and the oxirane groups both inter-molecularly and/or intra-molecularly to form a crosslinked network bound together by ester linking groups.
The baking steps used to cure the coating compositions of the invention may occur in discrete or combined steps. For example, substrates can be dried at ambient temperature to leave the coating compositions in a largely un-crosslinked state. The coated substrates can then be heated to fully cure the compositions to provide a hard coating. More preferably, the coating compositions of the invention are dried and heated in one step.
The temperature used in the baking process preferably ranges from about 60xc2x0 C. up to the decomposition temperature of the composition. Typically, baking at about 120xc2x0 C. to about 400xc2x0 C. for a period of time between about 3 seconds to about 15 minutes is sufficient to provide a fully cured composition, heat treatment at about 150xc2x0 C. to 220xc2x0 C. is preferred for about 1 minute to about 10 minutes.
The cured compositions of the present invention preferably have initial gloss values of at least about 90 when measured using a BYK Gardner micro gloss meter (catalog # 4520), and survive over 100 MEK Hammer rubs, when tested as described herein.
The cured coatings of the invention are particularly well suited as coatings for metal cans or containers. The containers may be coated with at least one layer of the cured coating, and the layers may be present on the inside of the containers, the outside of the containers, and the ends of the containers. The cured coatings adhere well to metal and provide substrates with high levels of resistance to corrosion or degradation that may be caused by food or beverage products.
The cured coatings also find excellent utility in the general packaging field, such as in the coating of aerosol cans. For example, the coating compositions of the present invention, in combination with a phenolic crosslinker and/or an amino resin, have successfully been tested inside a tin plated aerosol can. Such coatings have been found to provide outstanding resistance to dimethyl ether (DME) propellant in aerosol can applications. These coatings have also been found to be far superior to the presently commercially available coatings that are based on epoxy/phenolic crosslinker and/or amino crosslinker. They show, for example, improved flexibility, blush, and corrosion resistance. The also have the potential of reduced monomers/oligomers and reaction product migration into food.